The present invention relates to a method for controlling a Voltage Source Converter configured to convert direct voltage into alternating voltage and conversely and having at least one phase leg connecting two opposite poles of a direct voltage side of the converter and comprising a series connection of switching elements, each said element having at least two semiconductor devices of turn-off type, at least two free-wheeling diodes connected in parallel therewith and at least one energy storing capacitor, a mid point of said series connection forming a phase output being configured to be connected to an alternating voltage side of the converter and to divide the phase leg into an upper valve branch and a lower valve branch, said semiconductor devices of each switching element being controlled to obtain, for that switching element, one of a) a first switching state and b) a second switching state, in which for a) the voltage across said at least one energy storing capacitor and for b) a zero voltage, is applied across terminals of the switching element, for obtaining a determined said alternating voltage on said phase output, as well as an apparatus for converting alternating voltage into direct voltage and conversely according to the preamble of the appended independent apparatus claim.
The converter to be controlled may have any number of said phase legs, but it has normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof.
A voltage source converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage or conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage normally is converted into a three-phase alternating voltage and conversely or in so-called back-to-back-stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage. However, the present invention is not restricted to these applications, but other applications are also conceivable, such as in different types of drive systems for machines, vehicles etc.
A Voltage Source Converter of the type to be controlled through said method is known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and normally called a multi-cell converter or M2LC. Reference is made to these publications for the functioning of a converter of this type. Said switching elements of the converter may have other appearances than those shown in said publications, and it is for instance possible that each switching element has more than one said energy storing capacitor, as long as it is possible to control the switching element to be switched between the two states mentioned in the introduction.
The present invention is primarily, but not exclusively, directed to the control of a voltage source converter configured to transmit high powers, and the case of transmitting high powers will for his reason hereinafter be discussed for illuminating but not in any way restricting the invention thereto. When such a Voltage Source Converter is used to transmit high powers this also means that high voltages are handled, and the voltage of the direct voltage side of the converter is determined by the voltages across said energy storing capacitors of the switching elements and is normally set to be half the sum of these voltages. This means that a comparatively high number of such switching elements are to be connected in series or a high number of semiconductor devices are to be connected in series in each said switching element, and a Voltage Source Converter of this type is particularly interesting when the number of the switching elements is comparatively high, such as at least 8, and it may well be in the order of 20. A high number of such switching elements connected in series in said phase leg means that it will be possible to control these switching elements to change between said first and second switching state and already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters of the type shown in FIG. 1 in DE 101 03 031 A1 having switching elements with at least one semiconductor device of turn-off type and at least one free wheeling diode connected in antiparallel therewith. This makes it possible to obtain substantially lower switching losses and also considerably reduces problems of filtering of harmonic currents and radio interference, so that equipment therefor may be less costly.
A method of the type defined in the introduction is known through DE 101 03 031 A1 mentioned above and WO 2007/033852 A1. The latter describes a control method which is said to be a further development of the control method according to the former one. This control method starts from a synchronous control of switching the switching elements in the upper and lower valve branch for maintaining the same number of switching elements in each of said switching states in said series connection of switching elements between the direct voltage poles, and deviations therefrom is carried out for controlling valve branch currents with the aim to avoid balancing currents between the phase legs of the converter. These known control methods may in some situations have drawbacks, such as with respect to degree of complication and costs.